Workers will recognize that various techniques are known for confronting magnetic recording media with a magnetic recording transducer face. For instance, according to one technique, the transducer face is brought into contact with the passing medium. According to another technique ("non-contact" recording) the transducer face is virtually "flown" above the medium and kept out of contact therewith as a guard against damage to the medium, to the face or to both. Generally speaking, workers prefer such "out-of-contact" techniques where feasible. This invention concerns an improvement in "non-contact recording" and an associated novel configuring of the transducer face.
Magnetic memory storage units are a significant item of peripheral equipment in today's computers. In the typical unit data is stored on one or several magnetic disk drives. Such a drive will be recognized as characterized by one or more rotating magnetic recording surfaces on which data may be written, and read back, by a magnetic transducer mounted in a recording head. Such heads are "flown" in close proximity above a recording surface. Great care is taken that this flying head never "crashes" against the disk since catastrophic damage to both can result. Yet, to maximize recording density and optimize signal/noise, workers know that the "head spacing" (spacing between the head-face, and core there, and the surface of the moving disk) must be kept as small as possible and be held within very tight tolerances.
It is common to establish "head spacing" for a "flying head" by configuring the head-face in the fashion of an "air bearing" while establishing the proper fluid dynamics. The magnetic head surface is, today, mounted on a resilient suspension and urged toward the surface of the moving disk by a head actuation means, but is prevented from actual disk contact by an intervening cushion of air--called a "Bournoulli film" and established as the air bearing. Once this Bournoulli film is developed, it presents a rather substantial hydrodynamic resistance to reduction of "head-spacing" and significant force must be exerted to push the head closer to the disk. But certain abnormal conditions can disturb this "Bournoulli film" and suddenly remove it as a protective cushion, sending the head crashing into the disk. Thus, workers in the art are very meticulous in developing the proper (aerodynamic) headface configuration and in positioning the head so as to properly orient it (e.g., re pitch and roll angles) relative to the passing disk such close tolerances that a change of a minute of arc or so can be critical!
Workers know that it is critically important to maintain a predictable constant "head spacing" over a wide range of operating parameters if magnetic recording is to be successful. Head spacing is particularly critical with high density recording--e.g., it can vary the "fringing flux" pattern and affect read/write resolution.
The foregoing relates, mainly, to rigid media technology and--as workers are beginning to realize--is not necessarily applicable to flexible disks. This invention is particularly concerned with improved transducer configurations especially adapted for "near-approach" to floppy disk media during read/write sequences.
Workers also know that there are many factors affecting head spacing; such factors as the speed, configuration "penetration" and radial-position of a head [understand: "head speed" as the relative velocity between medium and transducer and "head penetration" as the penetration of the transducer stabilizer combination into the plane of the passing record medium, causing the latter to "dimple"]. Other affecting factors are disk characteristics (e.g., flexibility, thickness, etc.) and ambient conditions such as temperature and humidity.
Now, it is preferred here that a transducer be thrust to "penetrate" and "dimple" the flexible medium, and so better assure that head spacing be kept constant. However, such a "dimpling" can cause problems. For instance, it may degrade the desired film (Bournoulli) at the disk periphery and cause "flutter" there to upset the prescribed head spacing. Such problems have, to date, limited the useful recording area adjacent a disk's periphery, as workers well known. The present invention is adapted to help in maintaining constant head spacing by eliminating, or at least alleviating, the mentioned problems and so improving disk-head stability--particularly for "flying heads" adapted to transduce for a pack of floppy disks (rather than one disk). In such cases, there will be no "backing plate" (Bournoulli plate) as is typically used with a "single floppy" [e.g., IBM U.S. Pat. No. 4,074,330 mentions that a problem with such Bournoulli plates is that head spacing decreases as the head moves radially out on the floppy disk--and tries to solve this problem].
Now, the trend today is to record at ever higher "bit densities" (that is, to record individual data transitions that are closer together). And, as bit densities increase, one must reduce the "head spacing" more and more, as workers well know, (also, signal strength increases as head spacing drops). Thus, the task of configuring a head face to create the proper Bournoulli film becomes ever more critical with today's advanced high-bit-density equipment where head spacing on the order of just a few microinches is not uncommon.
This problem is greatly aggravated when one uses flexible disk media (floppy disks). As workers well know, it is not uncommon for such disks to develop surface undulations approximating several dozen microinches under high speed rotation.
Progress toward the more effective use of high speed flexible disks in recording systems is facilitated by a better understanding of stabilization requirements. Some workers, [see articles by R. Benson, D. Bogy in J. Appl. Mech., Vol. 45, p. 636 (1978); and by H. Greenberg, IEEE Trans., Vol. Mag-14, 5 (1978)] have studied the overall response to a localized load on a flexible disk. Greenberg (above) describes the head/disk interface with an expression that uses Reynolds equation for loading. This invention is directed toward establishing improved head surface geometry as a means of optimizing flying characteristics; and especially for providing stable, relatively uniform air bearing spacings in the sub-micron region at higher surface speeds (e.g., 40 m/sec.). It is an object of the present invention to develop a novel head configuration adapted to provide a proper "Bournoulli film" cushion when employed with flexible disks under high speed rotation, especially for transducing at high bit densities.
The use of flexible magnetic recording disks as a storage medium in an environment requiring high linear speeds has necessitated the design of air bearing contours which can provide reasonable wear characteristics at stable, sub-micron spacings. Unlike rigid disk sliders, these bearings must cope with the flexible nature of the disk as well as with the gas pressure forces which support it.
Several means for supporting a flexible disk in close proximity to a recording transducer have been discussed by workers in the art. I. Pelech and A. Shapiro [see J. Appl. Mech., Vol. 31, p. 577, 1964]; and P. Charbonnier [see IEEE Trans., Vol. MAG-12, 6, 1976] have discussed the possibility of a head fixed in a stationary plate, near which a flexible disk is rotating. The air film which develops between the plate and the disk serves to stabilize the disk in the axial direction. Charbonnier also suggested the use of a forced air nozzle to locally support the disk in the vicinity of a recording head.
The transducer may also be supported by an air bearing on the side of the disk opposite the stationary stabilizing plate. This approach facilitates radial motion of the head in order to access written tracks on the disk surface. The instabilities associated with the application of a stationary, localized load to a flexible disk supported in this manner have been analyzed by Benson and Bogy (article cited above), who provide a description of the disk response. Although this latter solution addresses the head/disk interface as well as the disk motion, the spacings developed are not adequate for high density digital recording.
This invention addresses the design of a suitable air bearing for use with a rotating flexible disk. The disk in this configuration is one of many co-rotating flexible mylar disks which are separated by thin spacers through which air is permitted to pump naturally, outward in the radial direction. The air bearing spacing is thus controlled by the pressure forces resulting from the self-acting gas film, opposed by the forces exerted by the disk and by the air flowing behind it. This bearing must maintain a uniform, stable, predictable spacing between the magnetic transducer and the media, while minimizing wear between the two adjacent surfaces. The dynamic stability of the disk must also be preserved.
Fixed head versus movable head:
It is conventional to design a computer disk file so that its flying recording heads are "movable" rather than "fixed". When operation begins and the disk surface is spun-up to the proper operating speed, the recording head is advanced, being pressed toward the disk to a "final float" position--close enough to generate the desired air bearing (Bournoulli film). Such a "flying head" may later be retracted when the disk is stationary (or rotated at low rpm) as desired (e.g., when "read/write" is completed). The technique used in bringing the head from "retracted" to "final float" position is commonly referred to as "landing" the head (even though there is no physical contact with the disk). This invention relates to "fixed" heads, and to techniques for promoting a more stabilized "final float" condition.
An example of a movable recording head is seen in U.S. Pat. No. 3,310,792 to Groom, et al. Here, a resilient gimbal spring is provided to suspend a magnetic recording head adapted to float on an air-film adjacent the surface of a rapidly moving memory disk. This gimbal spring can withdraw the head from the "float" position (e.g., see FIG. A) to "retracted" position (e.g., compare FIG. B) whereat the spring is in its neutral, or unstressed, condition. Advancing the head (e.g., via a driving pneumatic piston) back to "float" position, stresses the spring. Besides mounting the recording head for advancement and retraction, the gimbal spring also accommodates proper head orientation--exerting a very small moment on the head so that in "retracted" position, its "toe" (or leading edge) is further from the disk than its "heel" (or trailing edge)--whereas when in the "final float" position, the Bournoulli film developed will rotate the head somewhat so that its "heel and toe" are more nearly equidistant from the disk (compare FIGS. A and B).
Now, during "landing" there is danger of the heel contacting the disk, with the toe being pitched-up unless a significant Bournoulli film has been generated. In the past this problem has been addressed via a compromise between minimizing head spacing and optimizing read/write efficiency vs. emphasizing a "safe" landing mode (i.e., with too close a spacing, there is a high risk of "crash", whereas too great a spacing will degrade recording characteristics). This problem is addressed and, to an extent, solved by techniques taught in U.S. Pat. No. 3,678,482 to Billawala, discussed below.
Prior art head manipulation; FIGS. A, B:
FIG. A illustrates a typical prior art recording head 16 understood as "flying" adjacent a recording disk 17 in its "final float" position. (See also U.S. Pat. No. 3,678,482 for further details). As illustrated here, disk 17 will be understood as moving from right to left, with construction and operation being carried out conventionally except as otherwise specified. In its "final float" position, the recording head 16 floats with its heel end 19 at a minimum distance H from the surface of disk 17 and its face 21 tipped-away very slightly (or "pitched-up" by a very small angle .THETA.) so the forward projection of face 21 (i.e., bevel face 22 adjacent toe-end 18) is tipped slightly further away from disk 17, as known in the art. Face 21 is relatively flat and, being pitched-up slightly toward the approaching recording surface; merges into the second flat face 22 beveled away from the record and diverging from face 21 by a prescribed relatively small angle .alpha. (--the trailing edge of face 22 thus coinciding with the leading edge of face 21, as illustrated).
It should be recognized, of course, that angles .alpha. and .THETA. and distance h (as well as other like angles and distances set forth elsewhere herein) are greatly exaggerated for illustration purposes as compared with actual scale. Thus, for instance, in the prior art fluid film bearing illustrated in FIG. A distance h will preferably be on the order of a few dozen microinches and angles .alpha. and .THETA. on the order of a few minutes of arc.
As workers know it is conventional for a gimbal spring arrangement (not shown, but well known in the art) to be provided for suspending head 16 and permitting the indicated orientation--the head being thrust toward and away from record 17 by a piston arrangement 23 (not fully illustrated, but constructed and operated as well known in the art).
Under certain conditions head 16 is retracted (e.g., when disk rpm drops); conversely, the head may be advanced to "land" adjacent the disk for read/write operations when disk rpm reaches "operating speed". When piston 23 acts to so thrust head 16 it will be understood that a force P is applied and a moment M set-up to overcome the (rather slight) gimbal-spring-moment and rotate the head to fly more parallel with the passing disk surface (e.g., rotate head 16 from FIG. B to FIG. A orientation). A counter-moment M is then generated by the fluid film against both head faces; this made-up from a force F-1 acting through the center of pressure on the main face 21 and a force F-2 acting through the center of pressure on the bevel face 22. Thus, in "landing", it will be understood that pneumatic pressure applied via piston 23 forces the head toward disk 17 (--starting from the "retracted" position illustrated in FIG. B) to reduce angle .THETA. and eventually wind up in the "final float" position indicated in FIG. A. During "landing", it will be understood that the gimbal spring will tilt toe end 18 upward so that as head 16 is pressed toward disk 17, the heel 19 first approaches the disk--at this point a force F-1, originating from the fluid film, will commence to act on main face 21, pivoting heel 19 slightly away from record 17. (There being little or no fluid film pressure then applied upon bevel face 22 since it is still too remote from disk 17).
In the final phase of landing, it will be understood that the force distribution on head 16 is such that piston 23 must be located near the heel 19 (higher density air film then present). That is as the head approaches "final float" position and head spacing decreases, the point of application for piston 23 against head 16 moves closer to heel 19 (this maintaining the desired angle .THETA. for stable floating). With piston 23 positioned nearer heel 19, the landing moment is reduced (smaller moment arm, so F-1 is lower) and care must now be taken to avoid a "crash" against record 17 (heel 19 could strike disk 17 before sufficient balancing force F-1 is generated to pivot recording head into its final floating position.
The foregoing description relative to FIGS. A and B will serve to illustrate typical characteristics and problems associated with prior art, "non-rigid" heads and call associated difficulties to mind. Heads arranged according to this invention avoid all such difficulties since they are mounted to be rigid (non-movable) and arranged to induce the (flexible) medium to, alone, make the necessary approach and "landing". This should help workers to better appreciate the advantages and characteristics of a rigid-mounted "compound foil" head according to the invention--wherein no head positioning or alignment is required, but rather a simple presentation of the moving flexible disk so it will automatically position itself in proper transducing relation with the head core (as further discussed below). The operational advantages and the manifold difficulties avoided will be self-evident to workers familiar with this art.
Prior art: difficulties in "following" floppy disk with movable head FIG. 1A:
FIG. 1A shows very schematically, some of the factors involved in confronting a flexible magnetic recording medium, such as a floppy disk surface M, with a "canoe" type head A-1. Here, disk M will be understood as swept rotationally past the recording face of transducer head A-1 which is understood as mounted on a prescribed flexible suspension (indicated very generally as spring means A-2 and well known in the art). Head A-1 is shown as taking the form of the well known "canoe head" tilted up a prescribed pitch angle (aa) from coplanarity with an idealized perfectly level contact-plane R--R along which medium M (the confronting surface thereof) would ideally be swept. Medium M is given an exaggerated "wavy" configuration, since, as workers well know, it is very, very difficult to maintain such a flexible record surface flat along the prescribed plane R--R.
It will be recognized that in such an arrangement the head A-1 is "flown over the disk", its transducing face being urged compliantly towards reference plane R--R for transducing on the disk. Also, means will be understood as provided to urge the surface of head A-1 close to medium M, separated only by the Bournoulli air film of minute thickness, and to be very precisely maintained for accurate repeatable recording. As workers well know, so locating the head and moving floppy medium so close together and maintaining this precisely is akin to squeezing two spring-mounted foils together, as they pass one another at high speed--all in all a very unpleasant, somewhat imprecise, balancing act that can frequently go awry, with the result that read/write defects are introduced. This invention avoids such difficulties by postulating a relatively rigid transducer head over which the medium is made to fly--rather than flying the head over the medium!
One of the problems in prior art arrangements, like that of FIG. 1A, where the head is flown over the medium, is in making the "air bearing" (intervening film) stiff enough to maintain head-disk spacing despite changing factors. A related problem is to always maintain the pitch angle of the head constant relative to the approaching disk surface--and so mounting and driving the head mass that it closely "follows" the passing, undulating surface of a floppy disk. This, of course, is a very serious challenge since, when rotated at the usual high speeds, floppy disks can undulate and flop wildly. The frequency of these undulations is often so high that the head-following mechanism cannot readlly "follow" and maintain the tiny head-disk separation (as little as a few u-in.).
Another problem in so "following" floppy disks is that the head mass must really be made "ultra-light"--even so, it is difficult to find a spring system matched to the disk and to actuate it with the proper "following mechanism". Also, in any situation where a flexibly mounted head is arranged to follow a floppy disk surface, extreme difficulties can result from "feedback oscillations" often encountered. That is, an undulating disk can, for instance, cause sympathetic oscillations of the head. And the head, which typically has a different resonant frequency than the disk, will maintain these unwanted oscillations for some time before settling down--and may impact disks damagingly! On the other hand, using a rigid head, as in this invention, will lower the "Q" of the flexible medium and can damp-out such unwanted vibration.
The foregoing problems are exacerbated when operating a floppy disk in the desired "steep dive" mode whereby disk and head normally fly at a relatively large interspacing, with this being suddenly reduced as the head approaches the recording zone on the disk--then, the head dives steeply toward the disk (or vice versa) and is removed therefrom just as suddenly when the recording zone is passed (the head climbing steeply away from the disk). Such "steep dive" mode presents serious risks of damage to head, or to the disk or both, since a "crash" is much more likely under these conditions, with the stiffness of the air-bearing (Bournoulli film) varying widely the while. The present invention simplifies this situation greatly by making it unnecessary to "dive the head" toward the disk--rather it aerodynamically "sucks" and holds the disk (recording zone) closer to the rigidly-mounted head face (quasi "dash-pot" effect).
Invention features:
This invention avoids all the problems suggested above by mounting the head rigidly and inducing the passing disk segment to fly over the head in a prescribed manner. So doing, there is no need for spring-mounting the head, nor for related sensing and actuator mechanism to move the relatively large transducer mass. Rather, one simply develops aerodynamic forces that constrain the (relatively low mass) disk to "fly" at the proper height above the transducer core, with no need to "follow" the disk surface, as well as advantageously using a relatively "self-leveling" structure (the rotating floppy disk) to reference on a fixed stable surface (the rigidly-mounted transducer face).
Thus, as one feature of novelty, the present invention involves flying a flexible disk over a rigid head with the head face configured aerodynamically to induce a rather "steep mode" of confrontation, preferably, and involving a "compound curvature" configuration. (Or a small "lens" atop a large "lens"). In a related feature, a prescribed spherical head-foil is mounted atop a relatively flatter spherical mount-foil such that the stabilization-zone of the head-foil is kept within the (larger) stabilization-zone of the mount-foil, and more stabilized transducing promoted.